Conventional batch type plants for producing asphaltic mix typically comprise a drum dryer for heating and drying the stone aggregate, and a vertical bucket elevator for conveying the heated and dried aggregate from the dryer to the top of a tower. The tower includes an enclosed vibrating screen section at the top, for receiving and segregating the aggregate by groups of average particle size. The segregated aggregate then drops through individual chutes to a bin section which underlies the screen section, and which comprises a plurality of separate bins, and so that each bin receives an aggregate of a predetermined average size from the screen section. Four such bins are typically provided, with the initial or "sand" bin having greater capacity than the remaining bins. A weigh hopper is positioned below the bins, which permits the aggregate from one or more bins to be weighed out to provide a desired mix, as required for example for a roadway base coat or a finish coat. A mixing pugmill is positioned below the weigh hopper for mixing the weighed quantity of aggregate with hot liquid asphalt, and the bottom of the pugmill includes a gate opening, such that the contents may be dropped into an underlying truck or other receptacle.
When making an asphaltic mix containing recycle asphalt paving (RAP), cold RAP is conventionally delivered from a feed bin and into the elevator at a controlled feed rate, and so as to be delivered to the screen section concurrently with the heated aggregate. The aggregate is preferably superheated, to a temperature of at least about 400.degree. F., to permit it to dry and heat the RAP.
As will be apparent, once the decision is made to run RAP in the above described conventional plant, there is a fixed percentage of RAP in each bin, and all batches necessarily contain that percentage until the bins are emptied. Also, the percentage of RAP is limited to not more than about 20 percent, since a higher percentage tends to blind the screens of the screen section with sticky material.
In one early batch plant design, a separate bin for the cold RAP is positioned on the ground adjacent the tower, and a conveyor is provided for selectively delivering the cold RAP directly into the weigh hopper or into the pugmill mixer, where it comes into contact with superheated aggregate which is delivered from the storage bins. This results in a steam explosion, since the RAP typically has a high water content, and fumes and dust are blown into the atmosphere. Also, with this prior design, control of the composition of the final mix is limited, since the high temperature of the aggregate requires that RAP be included in each batch.
It is also known to add separate bins, which are filled by diverter chutes, to the side of a batch tower for special purposes. Some of these side bins are adapted to be filled with mixtures of virgin aggregate and RAP for blending with the aggregate contained in the other bins, but because of their position, these side bins often cannot supply all of the material for a batch. More particularly, these side bins load into one side of the weigh hopper, and the weigh hopper is thus unable to accept the desired volume of the mixture. Also, the side location of such bins renders it difficult to dispense RAP, since laterally directed side walls are required in order to direct the material laterally into the weigh hopper, and during discharge the RAP tends to stick to these side walls rather than flow smoothly from the bin.
It is accordingly an object of the present invention to provide an asphalt production plant which is adapted to process mixtures of aggregate, and/or aggregate and RAP, and which permits the formulation for each delivered batch to be readily varied, and which specifically permits formulations having a relatively high percentage of RAP in the mix of up to 30-40 percent.
It is a more particular object of the present invention to provide an asphalt batch plant which retains all of the versatility provided by a conventional multi bin tower, and which also has the further capability of providing mixes having a relatively high percentage of RAP with the segregated aggregates from any of the aggregate bins.
It is also an object of the present invention to provide an asphalt production plant which avoids the release of steam and other gases to the atmosphere.
It is a further object of the present invention to provide an asphalt production plant having a recycle bin which is positioned so as to permit a full charge of the recycle material to be freely delivered into the weigh hopper.